[disclosure I am a sanic contributor] aiohttp is a great project, very similar to sanic, I'd say the big thing is sanic is slightly more flask like than aiohttp in ergonomics (which is something aiohttp has been working on[1]).
Sanic also makes performance a key development goal so you can feel comfortable the project will scale gracefully. But I will stress I don't have current benchmarks to compare against aiohttp